1. Field of the Invention
The present invention relates to a wiring substrate which includes a wiring substrate body and a metallic plate serving as a stiffener or the like, as well as to the metallic plate itself. More particularly, the invention relates to a wiring substrate which includes a metallic plate formed from a rolled metallic sheet and serving as a stiffener or the like, as well as to the metallic plate.
2. Description of the Related Art
Conventionally, there is known a wiring substrate which includes a metallic stiffener bonded to a main surface of a wiring substrate body. Such a stiffener is used in instances wherein, due to low rigidity, the wiring substrate body easily deforms upon being subjected to an external force, or wherein the wiring substrate body easily deforms due to thermal expansion or other causes. The stiffener suppresses deformation of the wiring substrate and thus maintains the flatness of the wiring substrate, thereby increasing reliability of the wiring substrate.
An example of such a prior art wiring substrate is shown in FIGS. 9A and 9B, wherein FIG. 9A is an enlarged sectional view of a wiring substrate 220, and FIG. 9B shows a stiffener 201 for use in the wiring substrate 220. As illustrated, the wiring substrate 220 includes the stiffener 201, a wiring substrate body 211, and an IC chip 221. The stiffener 201 is a metallic plate of a predetermined rectangular shape formed from a rolled metallic sheet, and has a through-hole 202 formed at a central portion thereof so as to accommodate the IC chip 221.
The boundary profile or perimeter 211C of the wiring substrate body 211 has a rectangular shape substantially identical with that of the stiffener 201. The wiring substrate body 211 includes connection pads 212, which are formed on a first main surface 211A and which correspond to terminals 222 of the IC chip 221 to be mounted thereon, and electrode pads 215, which are formed on a second main surface 211B such that electrical continuity is established between the connection pads 222 and the corresponding electrode pads 215. The stiffener 201 is bonded to the first main surface 211A of the wiring substrate body 211 using an adhesive layer 217 such that the profile 211C of the wiring substrate body 211 is in substantial alignment with the profile 201 C of the stiffener 201. The IC chip 221 is mounted on the first main surface 211A in such a manner as to be accommodated within the through-hole 202 formed in the stiffener 201. The terminals 222 of the IC chip 221 are soldered to the corresponding connection pads 212.
In some cases, a space defined by the IC chip 221 and the first main surface 211A of the wiring substrate body 211 is filled with underfill resin so as to improve reliability of connection between the IC chip 221 and the connection pads 212. In some cases, a heat radiation plate with or without fins is attached to the stiffener 201 and the IC chip 221 in order to effectively radiate heat generated by the IC chip 221.
However, when the stiffener 201 is bonded to the wiring substrate body 211 in the course of manufacture of the wiring substrate 220, the wiring substrate 220 may suffer deformation, such as warpage or waviness. This conceivably results from a difference in coefficient of thermal expansion between the stiffener 201 and the wiring substrate body 211 or from a small variation in coefficient of thermal expansion within the wiring substrate body 211.
In the case where the IC chip 221 is mounted on the wiring substrate body 211 after the stiffener 201 is bonded to the wiring substrate body 211, such a deformation may cause failure in one or more of the connections between the terminals 222 of the IC chip 221 and the corresponding connection pads 212 of the wiring substrate body 211, thereby resulting in an impairment in the yield of acceptable devices produced by the corresponding manufacturing process. In the case where the stiffener 201 is bonded to the wiring substrate body 211 after the IC chip 221 is mounted on the wiring substrate boy 211, a strong stress force is produced which acts on connections between the terminals 222 of the IC chip 211 and the connection pads 212 of the wiring substrate body 211, potentially resulting in cracking in one or more of these connections and a resultant electrical defect. When the wiring substrate 220 is mounted on another printed wiring board, the connection provided between the printed wiring board and the electrode pads 215 of the wiring substrate 220 may be unacceptable, again resulting in an impairment or reduction in yield.
The warpage or waviness of the wiring substrate 220 varies with temperature. Accordingly, when the process of mounting the IC chip 221 is repeatedly started and stopped, i.e., when the wiring substrate 220 is repeatedly subjected to cooling and heating cycles, stress forces acting on connections between the terminals 222 of the IC chip 221 and the corresponding connection pads 212 of the wiring substrate body 211 can cause progressive metal fatigue over time in the connection(s), resulting in cracking in the connection(s) and a resultant electrical defect.
One aspect of the present invention concerns the discovery or inventive appreciation by the present inventors that deformation of a wiring substrate such as that shown at 220 does not arise isotropically, but rather tends to arise intensively in a given direction. Specifically considering the prior art device described above, when the stiffener 201 is bonded to the wiring substrate body 211 such that the short sides of the wiring substrate body 211 of rectangular shape are oriented in substantially the same direction as the rolling direction of the stiffener 201, the amount of deformation of the wiring substrate 220 is minimized. In contrast, when the stiffener 201 is bonded to the wiring substrate body 211 such that the diagonal direction of the wiring substrate body 211 is parallel with the rolling direction of the stiffener 201, the amount of deformation of the wiring substrate 220 is maximized. Accordingly, by studying the relationship between the rolling direction of the stiffener 201 and the shape of the wiring substrate body 211 and that of the stiffener 201, the amount of deformation of the wiring substrate 220 can be controlled.
The present invention is based on the findings of the inventors discussed above and an object of the invention is to provide a wiring substrate which is equipped with a metallic plate serving as, for example, a stiffener, and which is constructed so as to reduce or suppress deformation which arises in the course of manufacture, and deformation due to temperature variations during operation. Another object of the present invention is to provide the metallic plate serving as, for example, a stiffener.
To achieve the foregoing objects, the present invention provides a wiring substrate comprising a wiring substrate body having a main surface; and a metallic plate formed from a rolled metallic sheet and having a rolling direction, the metallic plate having a boundary profile or outline, as viewed in plan, which is substantially identical to, or smaller than, that of the wiring substrate body, and the metallic plate being bonded to the wiring substrate body within the main surface. When two parallel lines are in contact with and enclose therebetween the boundary profile or outline of the metallic plate (hereinafter such parallel lines will be described as simply xe2x80x9cenclosingxe2x80x9d the profile) in such a manner as to maximize the perpendicular distance between these lines, a first direction of the profile of the metallic plate extending perpendicular to the two parallel lines intersects the rolling direction of the metallic plate at an angle. Preferably, the metallic plate serves as a stiffener for reinforcing the wiring substrate body. However, the metallic plate may be a heat sink, a metal lid, or any other functional element.
Being formed from a rolled metallic sheet, the stiffener exhibits anisotropic rigidity, meaning that the stiffener deforms more easily in its rolling direction than in a direction perpendicular to this rolling direction. As mentioned previously, when the stiffener is bonded to the wiring substrate body, stress may be produced because of fine differences in the coefficient of thermal expansion between the stiffener and the wiring substrate body. Accordingly, the wiring substrate significantly warps or becomes wavy in the rolling direction of the stiffener, in which the bonded stiffener deforms relatively easily. In contrast, the wiring substrate warps less or becomes less wavy in a direction perpendicular to the rolling direction, i.e., the direction in which the stiffener deforms less easily. Thus, when the first direction of the boundary profile of the stiffener matches the rolling direction, i.e., when the first direction of the profile of the stiffener extends parallel with the rolling direction, the dimension of the stiffener is the longest in the rolling direction (i.e., in the first direction), the direction in which the stiffener is more likely to deform. Therefore, the wiring substrate warps or becomes wavy along this longest dimension and consequently, the amount of deformation of the wiring substrate is of a maximum. For example, in the case of a rectangular stiffener, when its diagonal direction, (i.e., the direction defined by a line between opposed comers of the rectangle) matches, i.e., extends in the same direction as or parallel to, the rolling direction, the amount of deformation of the wiring substrate is at a maximum. Accordingly, when the first direction of the profile of the stiffener intersects the rolling direction at an angle rather than matching or extending parallel thereto, the amount of deformation of the wiring substrate can be reduced.
Reduction or suppression of the deformation of the wiring substrate improves the reliability of connections made between the wiring substrate and an electronic component mounted thereon, as well as the reliability of connections made between the wiring substrate and another printed wiring board on which the wiring substrate is mounted. Accordingly, in manufacture of the wiring substrate or in the connection of the wiring substrate to another printed wiring board, yield is improved because of the improved reliability of the wiring substrate.
The amount of deformation, such as warpage or waviness, caused by temperature variations to which the wiring substrate is subjected, is also reduced as compared with a case wherein the first direction of the profile of the stiffener extends parallel to the rolling direction of the stiffener. Consequently, even when the wiring substrate undergoes repeated cooling and heating cycles associated with the repeated start-stop operations of an electronic component mounted thereon, there is a reduction of the potential fractures in the connections between the associated electronic component and the wiring substrate body or between the wiring substrate and another printed wiring board, as well as in associated electrical defects.
As used herein, the xe2x80x9cfirst directionxe2x80x9d of a boundary profile or region denotes a direction perpendicular to two parallel lines that enclose therebetween the profile or region in such a manner as to maximize the distance between the parallel lines. When a particular profile or region involves a plurality of cases where the distance between the two enclosing lines is maximized, as in the case of a rectangular profile, any one of the cases can be selected in defining the first direction. The xe2x80x9csecond directionxe2x80x9d of a profile or region denotes a direction perpendicular to two further parallel lines that enclose therebetween the profile or region in such a manner as to minimize the distance between the further lines.
A dimension of a profile or region that extends along the rolling direction denotes or refers to a dimension defined by the perpendicular distance between two parallel lines that are perpendicular to the rolling direction and enclose therebetween the profile or region.
The wiring substrate body may have any form so long as the body includes an insulating layer and a wiring layer. Examples of such a wiring substrate body include a laminated wiring substrate body having a plurality of insulating layers and wiring layers arranged in an alternating manner on a single side, or on opposite sides, of a core substrate. The wiring substrate body does not necessarily include a core substrate. The insulating layer may be made of a number of different materials including the following: a resin, such as an epoxy resin, a polyamide resin, a BT resin, or a PPE resin; a composite material, such as a composite of any one of the above-mentioned resins and glass fiber (glass woven fabric or glass unwoven fabric) or a composite of any one of the resins and organic fiber, such as polyamide fiber; a resin-resin composite material formed by impregnating a three-dimensional network fluorine-containing resin base material, such as continuously porous PTFE, with a resin, such as an epoxy resin; or a ceramic, such as alumina.
The wiring substrate body may include terminals, such as connection pads or bumps, formed on the main surface thereof for connection thereto of an IC chip or any other electronic component. For example, when an IC chip is to be mounted on the wiring substrate body, the connection pads or bumps may be arranged in a lattice or an ordered array in many cases, but are not necessarily arranged regularly in a lattice. The wiring substrate body may include terminals, such as connection pads or bumps, formed thereon or upright pins arranged thereon, for connection to another printed wiring board.
The boundary profile or outline of the wiring substrate body may have a substantially rectangular shape or form (including a substantially square form), a polygonal form, or any other form in accordance with the objectives of the invention as described herein.
The material used for the stiffener may be selected as appropriate, taking into consideration such factors as rigidity and the coefficient of thermal expansion. The stiffener may be formed from a rolled sheet of, for example, copper, copper alloy, aluminum, or stainless steel. Such a metallic sheet may be plated with, for example, nickel or gold.
When the coefficient of thermal expansion of the stiffener is greatly different from that of the wiring substrate body, the resultant wiring substrate becomes more likely to deform. Therefore, a small difference in coefficient of thermal expansion is preferred.
The boundary profile or outline of the stiffener may have a substantially rectangular shape or form (including a substantially square shape or form), a polygonal form, or any other form, in accordance with the objectives of the invention. The profile or outline of the stiffener is not necessarily similar to that of the wiring substrate body. In order to mount an IC chip or any other electronic component on the wiring substrate body, the stiffener may have a corresponding through-hole formed therein.
The material for an adhesive layer used for bonding the stiffener and the wiring substrate body may be selected as appropriate in consideration of such factors as the adhesion thereof to the stiffener and the wiring substrate body, the bonding temperature, and the heat resistance of the wiring substrate body. An adhesive in paste form or film form may be used in forming the adhesive layer.
The wiring substrate may include not only the wiring substrate body and the stiffener but also an IC chip or any other electronic component. A plurality of electronic components may be mounted on the wiring substrate.
Preferably, in the above-described wiring substrate, when two parallel lines enclose therebetween the profile or outline of the stiffener in such a manner as to minimize the distance therebetween, the second direction of the profile or outline of the stiffener perpendicular to the two lines substantially matches the rolling direction of the stiffener.
Because the second direction of the profile or outline of the stiffener substantially matches the rolling direction of the stiffener, in the case of, for example, a substantially rectangular stiffener, the direction of the short sides of the rectangular stiffener substantially matches the rolling direction. Accordingly, the dimension of the profile or outline of the stiffener along the rolling direction is the shortest dimension thereof, thereby minimizing the amount of deformation of the wiring substrate associated with bonding of the stiffener to the wiring substrate body and that associated with temperature variations to which the substrate is subjected. Therefore, the wiring substrate can very significantly improve the reliability of a connection or connections made to an electronic component mounted thereon as well as the reliability of a connection or connections made to another printed wiring board.
Preferably, the wiring substrate comprises a wiring substrate body having a main surface and a substantially rectangular shape or form as viewed from above; and a stiffener formed from a rolled metallic sheet, and the profile or outline of the stiffener is substantially identical to that of the wiring substrate body and is thus rectangular as well, the metallic plate being bonded to the main surface of the wiring substrate body and the profile or outline of the stiffener is substantially aligned with that of the wiring substrate body. The diagonal direction of the stiffener intersects the rolling direction of the stiffener at an angle.
Because the diagonal direction of the profile or outline of the stiffener intersects the rolling direction of the stiffener at an angle, the dimension of the stiffener along the rolling direction is shorter than that along the diagonal direction. Accordingly, the stiffener is less likely to deform than a stiffener in which the diagonal direction of the profile or outline matches the rolling direction. Because the amount of deformation of the wiring substrate associated with bonding of the stiffener to the wiring substrate body and that associated with temperature variations can be significantly reduced, the resultant wiring substrate can improve the reliability of a connection or connections made to an electronic component mounted thereon as well as the reliability of a connection or connections made to another printed wiring board.
Preferably, the direction of short sides of the rectangular stiffener substantially matches the rolling direction of the stiffener. Because the direction of short sides of the profile or outline of the stiffener substantially matches the rolling direction of the stiffener, the dimension of the stiffener along the rolling direction is the shortest dimension. Thus, the amount of deformation of the wiring substrate associated with bonding of the stiffener to the wiring substrate body and that associated with temperature variations can be reduced or suppressed. Therefore, the wiring substrate can improve the reliability of a connection or connections made to an electronic component mounted thereon as well as the reliability of a connection or connections made to another printed wiring board.
The present invention further provides a metallic plate for the above-described wiring substrate. The metallic plate is cut from a rolled metallic sheet such that the first direction of the profile or outline of the metallic plate intersects the rolling direction of the metallic plate at an angle.
In the stiffener of the present invention, the dimension along its rolling direction is shorter than that along its first direction. Thus, the stiffener is less likely to warp or become wavy than one in which the first direction of the profile or outline substantially matches its rolling direction. Through use of the stiffener of the present invention, manufacture of a wiring substrate enables reduction or suppression of deformation of a wiring substrate body associated with bonding of the stiffener to the wiring substrate body, meaning that the deformation of the resultant wiring substrate is also reduced or suppressed. In addition, the amount of deformation of the wiring substrate associated with, i.e., resulting from, temperature variations experienced by the substrate is also reduced or suppressed. Therefore, the wiring substrate of the invention can improve the reliability of a connection or connections made to an electronic component mounted thereon as well as the reliability of a connection or connections made to another printed wiring board.
Preferably, the second direction of the profile or outline of the stiffener substantially matches the rolling direction of the stiffener. In a stiffener of this construction, because the second direction of the profile or outline substantially matches its rolling direction, the dimension along the rolling direction is the shortest dimension. According, through use of this stiffener, manufacture of a corresponding wiring substrate results in minimization of deformation of the wiring substrate associated with bonding of the stiffener to a wiring substrate body as well as minimization of deformation of the wiring substrate associated with temperature variations to which the substrate is subjected. Therefore, the wiring substrate can dramatically improve the reliability of a connection or connections made to an electronic component mounted thereon as well as the reliability of a connection or connections made to another printed wiring board.
The present invention still further provides a wiring substrate comprising a wiring substrate body having a main surface; and a metallic plate formed from a rolled metallic sheet, the profile or outline of the metallic plate being substantially identical with or larger than that of the wiring substrate body, and the main surface of the wiring substrate body being bonded to the metallic plate within the profile or outline of the metallic plate. When two parallel lines enclose therebetween the profile or outline of the wiring substrate in such a manner as to maximize the perpendicular distance between the lines, a first direction of the outline of the wiring substrate body perpendicular to the two lines intersects the rolling direction of the metallic plate at an angle. Preferably, the metallic plate serves as a stiffener for reinforcing the wiring substrate body. However, the metallic plate may be a heat sink, a metal lid, or any other functional element.
As mentioned previously, the stiffener is more likely to deform in its rolling direction. It is also noted that the portion of the wiring substrate where the stiffener and the wiring substrate body are bonded together suffers stress and deformation. Accordingly, when the profile or outline of the metallic plate is substantially identical to, or larger than, that of the wiring substrate body, the deformation of the wiring substrate varies greatly according to the relationship between the profile or outline of the wiring substrate body and the rolling direction of the stiffener.
When the first direction of the profile or outline of the wiring substrate body intersects the rolling direction of the stiffener at an angle, deformation of the wiring substrate occurring during the course of manufacture and deformation due to temperature variations to which the substrate is subjected can be reduced or suppressed. Thus, the wiring substrate of the present invention can improve the reliability of a connection or connections made to an electronic component mounted thereon as well as the reliability of a connection or connections made to another printed wiring board. Accordingly, the overall fabrication yield can be improved, and a wiring substrate of high reliability can be obtained.
Preferably, in the wiring substrate described above, when two parallel lines enclose therebetween the profile or outline of the wiring substrate body in such a manner as to minimize the distance therebetween, the second direction of the profile or outline of the wiring substrate body perpendicular to the two lines substantially matches the rolling direction of the stiffener. Since the second direction of the wiring substrate body substantially matches the rolling direction of the stiffener, the dimension of the wiring substrate body along the rolling direction is the shortest dimension, thereby minimizing the amount of deformation of the wiring substrate associated with bonding of the stiffener to the wiring substrate body and associated with temperature variations. Therefore, the wiring substrate can dramatically improve the reliability of a connection or connections made to an electronic component mounted thereon as well as the reliability of a connection or connections made to another printed wiring board.
The present invention still further provides a wiring substrate comprising a wiring substrate body having a main surface, which includes an IC-chip-mounting region and a metallic-plate-bonding region arranged to substantially surround the IC-chip-mounting region; and a metallic plate formed from a rolled metallic sheet and having a hole corresponding to the IC-chip-mounting region formed therein, the metallic plate being bonded to the metallic-plate-bonding region. When two parallel lines enclose therebetween the IC-chip-mounting region in such a manner as to maximize the distance between the lines, the first direction of the IC-chip-mounting region perpendicular to the two lines intersects the rolling direction of the metallic plate at an angle. Preferably, the metallic plate serves as a stiffener for reinforcing the wiring substrate body. However, the metallic plate may be a heat sink, a metal lid, or any other functional element.
Deformation, such as warpage or waviness, of the wiring substrate may result in a failure to reliably mount an IC chip on the wiring substrate. Further, when the wiring substrate is subjected to cooling or heating after the IC chip is mounted thereon, the amount of warpage or waviness of the wiring substrate varies, thereby causing stress in the connection terminal(s) and potentially resulting in fracture of the connection terminal(s). In this regard, it is noted that the connection terminals that are most distant from the remaining terminals are most susceptible to stress and thus fracture. Through the reduction or suppression of warpage or waviness of the wiring substrate in areas between such connection terminals and other terminals and the reduction or suppression of variations of the warpage or waviness associated with cooling or heating, the connection terminals become less susceptible to fracture, thereby improving connection reliability.
According to the present invention, the first direction of the IC-chip-mounting region intersects the rolling direction of the metallic plate at an angle. Thus, the wiring substrate can reduce or suppress the amount of deformation of the IC-chip-mounting region as compared with a substrate in which the first direction of the IC-chip-mounting region substantially matches the rolling direction of the stiffener. Further, the wiring substrate can reduce or suppress the amount of deformation of the region associated with temperature variations.
Accordingly, when an IC chip is mounted on the wiring substrate body after the stiffener is bonded to the wiring substrate body, the terminals of the IC chip can be reliably connected to the corresponding connection pads or bumps on the wiring substrate body. When the stiffener is bonded to the wiring substrate body after the IC chip is mounted on the wiring substrate body, little stress occurs in the connections, so that the connections are not susceptible to fracture. Since the amount of deformation of the wiring substrate is small when the wiring substrate is subjected to cooling and heating cycles, i.e., to cyclic temperature variations, the connections are less susceptible to fracture, thereby improving the connection reliability.
Preferably, in the above-described wiring substrate, when two parallel lines enclose therebetween the IC-chip-mounting region in such a manner as to minimize the distance between the lines, the second direction of the IC-chip-mounting region perpendicular to the two lines substantially matches the rolling direction of the stiffener. Because the second direction of the IC-chip-mounting region substantially matches the rolling direction of the stiffener, the dimension of the IC-chip-mounting region along the rolling direction is the shortest dimension, thereby minimizing the amount of deformation of the IC-chip-mounting region associated with bonding of the stiffener to the wiring substrate body and that associated with temperature variations. Therefore, the wiring substrate can dramatically improve the reliability of connection thereof to the IC chip mounted thereon.